(a) Field of the Invention
The present invention relates to a power factor correction (PFC) controller. More specifically, the present invention relates to a boundary mode PFC controller using a boost converter.
(b) Description of the Related Art
FIG. 1 shows a schematic of conventional boundary mode PFC controller using a boost converter, comprised of a boost converter 10 and a switching controller 20. The boost converter 10 rectifies input AC power and generates rectified power voltage Vs, and outputs a voltage Vout to a load part according to the operation of a switch (a metal oxide semiconductor field effect transistor or MOSFET). The switching controller 20 controls the switch of the boost converter 10. The switching controller 20 detects a fluctuation of input voltage or output voltage and controls the switch, such that the output voltage of the boost converter 20 is regulated to a specified value.
Referring to FIG. 1, when input voltage is assumed to be increased, the voltage Vm1, divided from the voltage Vs by the resistors R3 and R4, is increased, and accordingly, the output voltage Vmo of a multiplier 23 is also increased. Therefore, the point at which the voltages Vmo and Vcs provided to a comparator becomes equal is delayed, and hence the ON interval of the switch is increased. Hence, the output voltage Vout of the boost converter 10 is increased.
When the output voltage Vout of the boost converter 10 is increased, output voltage Vm2 of the error amplifier 21 is reduced, and an output voltage Vm2-Vref of subtracter 21 is reduced. Hence, the output voltage Vmo of the multiplier is reduced.
In the boundary mode PFC controller, when the input voltage is increased, the multiplier output voltage Vmo is momentarily increased, but the subtracter output voltage Vm2-Vref provided to the multiplier 23 is reduced due to the increase in the boost converter 10 output voltage, and the multiplier output voltage Vmo is reduced. Therefore, the multiplier output voltage Vmo continues to be constant, independent of the increase in input voltage, and the boost converter 10 output voltage Vout is regulated.
That is, as shown in FIG. 2, when the input voltage is increased and the voltage Vm1 is increased to Vm1', the output voltage Vm2 of the error amplifier 21 is reduced to Vm2', and eventually, the output voltage Vmo of the multiplier 23 is maintained to be constant.
However, in the prior art boundary mode PFC controller, when the input voltage is highly increased more than a specified voltage, the output voltage Vm2 of the error amplifier is increased to be higher than the reference voltage Vref, and the output voltage of boost converter 10 is no longer regulated. That is, in the prior art, the fluctuation range of the input voltage is limited by the reference voltage Vref.